1. Field of the Invention
The present invention relates to an ultrasonic distance measuring apparatus for producing data on a distance to an object by transmitting and receiving an ultrasonic wave, or more in particular to an ultrasonic wave distance measuring apparatus comprising means for detecting variations in the humidity and temperature of the atmosphere and means for compensating for variations in the propagation characteristics of the ultrasonic wave caused by the variations in temperature or humidity.
2. Description of the Prior Art
Generally, the ultrasonic wave propagating in the atmosphere is attenuated in proportion to the distance to an object. The amount of attenuation, however, depends to a considerable measure on the characteristics such as temperature and humidity of the atmosphere. Taking as a reference level of the received signal the level at the temperature of 20.degree. C. and the humidity of 50%, for example, the received signal level undergoes the variations as shown in FIGS. 1(a) and 1(b) by change of temperature and humidity respectively.
Now, let us discuss modes that variations in the atmospheric characteristics affect the operation of an ultrasonic distance measuring apparatus.
An ultrasonic distance measuring apparatus using an ultrasonic wave, which is implemented to produce distance data from the period of time required from the transmission of the ultrasonic wave transmitted toward an object to the reception of the wave reflected on the object and must always perform the operation of detecting whether or not the reflected signal has been received and detect and set the receiving time. Generally, this purpose is attained in such a way that the received signal is compared with a reference level set above a noise level, and whenever the level of the received signal exceeds the reference level, it is decided that the reflected wave is received at that time point. In the even that the level of the received signal changes with change in the characteristics of the atmosphere as described above, however, the time point when the reflected wave is assumed to have been received by the detecting operation naturally undergoes a change, with the result of changing the time length from the transmission to the reception of the ultrasonic wave.
The variation in the time length from the transmission to the reception of an ultrasonic wave results in changing distance data with change in the temperature or humidity despite that the object remains at a predetermined fixed distance from the distance measuring apparatus. This poses a great problem of the impossibility of producing accurate distance data.
In other words, the variation in the level of the received signal due to variation in the characteristics of the atmosphere brings a great adverse effect on the accuracy of the distance measuring operation of an ultrasonic distance measuring apparatus.